1. Field of the Invention
This invention relates to electrical devices comprising conductive polymer compositions and to a method or attaching power leads to conductive polymer elements.
2. Discussion of the Prior Art
Electrical devices, such as for example, heaters and current limiting devices, comprising conductive polymer compositions are described in the literature and are commercially available.
In such devices a conductive polymer composition is attached in some manner to a source of electrical power. This is generally provided by what is referred to in the art as an electrode which is in contact with the conductive polymer composition and which is connected to a source of electrical power. One type of electrode that can be used with conductive polymer compositions is a wire mesh or grid electrode at least partially embedded in the conductive composition. The grid or mesh must then be attached to a power lead in some fashion. Several techniques for attaching such leads have been suggested. One method comprises soldering or welding power leads to a portion of the mesh which extends beyond the edge of the conductive polymer element. Because of this extension, production of heaters by a continuous extrusion process is limited to substantially rectangular heaters. Also, protruding edges of the mesh electrode are difficult to cut to the desired size. Attachment of a power lead to the center of the mesh by welding or soldering creates a protrusion on the planar surface of the heater making direct contact between the entire heater surface and the surface of the object to be heated difficult or impossible to maintain.
A discussion of attaching electrodes to conductive polymer compositions can be found in U.S. Pat. No. 3,351,882 (Kohler et al.) and U.K. Pat. No. 1,167,551 (Texas Instruments). In the U.K. patent, perforated electrodes are maintained in good electrical contact with a PTC element of a heater by an insulating sleeve. The material of the sleeve and PTC element coalesce in the perforations to maintain this contact. Electrical leads can be connected to the electrodes by any suitable manner, as by peeling away a portion of the outer jacket and soldering the leads to the perforated electrode strips. As mentioned above, soldering leads to electrodes of this type is frequently undesirable.